THORAX AND ABDOMEN OF THE HORSE 53 



The union of each costal cartilage with the sternum is reinforced by 

 a radiate sterno-costal ligament (ligamentum sternocostale radiatum), 

 of considerable strength and triangular outline, placed on the inner side 

 of the joint. 



Interchondral ligaments. — In general, the cartilages of the 

 asternal ribs are attached to each other by elastic tissue ; but the 

 eighth and ninth are more firmly united by strong fibrous tissue. A 

 band, the cho7idro- xiphoid ligament (ligamentum chondroxiphoidea), 

 passes from the ninth cartilage to the xiphoid cartilage. 



Intersternal synchondroses^ (Synchondroses intersternales). — 

 In the animal at birth the individual bony segments of the sternum 

 are united by cartilage, which, with advancing age, becomes more and 

 more invaded by ossification, until in old animals the various segments 

 may be fused into one bony mass. The coalescence of the sternebrae 

 begins with the last two, and gradually extends in a cranial direction. 



An internal proper sternal ligament (ligamentum sterni proprium 



internum) runs along the inner surface of the sternum. It begins as a 



narrow single band, just behind the joint between the first rib and the 



sternum, where it is connected with strong fibrous tissue binding the 



first pair of ribs together, but soon divides into three limbs. The two 



lateral limbs disappear on the seventh or eighth costal cartilages ; Avhile 



the medial limb can be followed along the sternum to the xiplioid 



process. 



Dissection. — Clean the thoracic part of the aorta and the venaazygos, 

 and do so with care, otherwise the thin-walled thoracic duct, which lies 

 between the aorta and the vein, will be destroyed. 



Aorta thoracica. — The thoracic part of the aorta continues the 

 aortic arch. At first to the left of the median plane, it gradually 

 inclines towards the right and gains the middle line before the 

 diaphragm is reached. Leaving the thorax by the hiatus aorticus, it 

 enters the abdomen and is thereafter distinguished as the abdominal 

 aorta. 



Close to its commencement, the aorta crosses the left face of the 

 oesophagus. To the right it is also related to the vena azygos and the 

 thoracic duct. Ventral to the aorta is the root of the left lung. 



The branches of this part of the aorta are as follows :— 



(1) Trunciis hronchooasophageus. — The broncho-oesophageal trunk 

 is a short single stem that leaves the aorta about the sixth thoracic 

 vertebra, descends over the right face of the parent vessel, and soon 

 divides into the bronchial and cesophageal arteries. Occasionally the 

 trunk leaves the aorta in common with the first pair of aortic inter- 

 1 avv (syn) [Gr.], together. Xd^/Spos (chondros) [Gr.], cartilage. 



